1. Technical Field
Various embodiments of the present disclosure relate generally to photomasks and, more particularly, to a method of fabricating a reflective photomask.
2. Related Art
Integration density of semiconductor devices has been continuously increased with the development of nano-technology. As the semiconductor devices become more highly integrated, some elements such as transistors and interconnection lines have been scaled down to reduce their sizes. Next-generation semiconductor technologies may depend on process technologies, which provide the possibility of formation of fine patterns, rather than circuit design technologies. Formation of the fine patterns typically involves use of photolithography process technologies. Generally, a photolithography process may include forming a photoresist layer on a semiconductor substrate, transferring desired pattern images onto the photoresist layer using a light to selectively expose portions of the photoresist layer, and developing the exposed photoresist layer with a developer to form photoresist patterns. The light used in the exposure step may directly affect the resolution of the photoresist patterns.
For example, the resolution of the photoresist patterns may depend on the wavelength of the light used in the exposure step of the photolithography process. Various lights may be used in the exposure step of the photolithography process. For example, the light used in the exposure step of the photolithography process may be one of an i-line beam having a wavelength of about 365 nanometers, a krypton fluoride (KrF) laser beam having a wavelength of about 248 nanometers, or an argon fluoride (ArF) laser beam having a wavelength of about 193 nanometers. Even though lights having short wavelengths, such as the i-line beam, the KrF laser beam and the ArF laser beam are used in the exposure step of the photolithography process, there may be still limitations in enhancing the pattern resolutions of the highly integrated semiconductor devices. Thus, in recent years, extreme ultraviolet (EUV) rays referred to as soft X-rays have been revealed to overcome the limitations of the photolithography processes utilizing the i-line beam, the KrF laser beam or the ArF laser beam as a light source.
An EUV ray has a wavelength of about 13.5 nanometers which is considerably less than those of the i-line beam, the KrF laser beam or the ArF laser beam. Since the wavelength of the light used in the exposure step may directly affect the pattern resolution, an EUV lithography process utilizing an EUV ray as a light source may exhibit a remarkably higher pattern resolution than a photolithography process utilizing an i-line beam, a KrF laser beam, or an ArF laser beam as a light source. However, an EUV ray has also higher energy because the wavelength of the EUV ray is shorter than an i-line beam, a KrF laser beam, or an ArF laser beam. Thus, it may be difficult to use conventional transmission-type photomasks in an EUV lithography process. This is because transparent substrates of conventional transmission-type photomasks are too weak to endure the high energy of the EUV ray and the energy efficiency of the EUV ray is too low to enhance the pattern resolution. Accordingly, reflective photomasks have been proposed to overcome the disadvantages of the typical transmission-type photomasks. When the reflective photomasks are used in an EUV lithography process, the EUV ray may be reflected on surfaces of the reflective photomasks without penetrating the reflective photomasks. Hence, the high energy of the EUV ray may not be applied to the substrates of the reflective photomasks.